Embodiments of the invention relate generally to true time delay (TTD) beam formers for an electrically steerable array antenna or phased array antenna, and more particularly to TTD beam formers including TTD modules incorporating radio frequency (RF) micro-electromechanical systems (MEMS) switches controlled by a combination of make-before-break and sparse array methodologies.
Electronically steered antenna (ESA) systems or phased array antenna (PAA) systems combine the signals from multiple stationary antenna elements to point a beam of radio waves at a certain angle in space. The characteristics and angle of the beam is controlled in a manner that electronically steers the beam in different directions without physically moving the antennas. The electronic beam steering in a phased array antenna is often accomplished in one of two ways: through the use of phase shifters or true time delay devices. TTD beam steering differs from a phase shifter type approach in the inherent bandwidth of the device and the fact that the device imparts a time delay rather than a phase shift. These distinctions allow the TTD device to be used in very wideband applications for forming antenna beams and nulls. This is advantageous for electronic warfare systems and broadband communication applications.
Beam steering via TTD is accomplished by changing the excitation time of each antenna element. A TTD module is fabricated with high speed switches coupled to transmission lines of various lengths. The amount of time it takes for a signal to be transmitted between the electronics and the antenna is controlled by selecting a particular combination of transmission lines, which imparts a desired amount of time delay on the RF signal. Selection of the transmission lines may be accomplished using different types of switching elements such as RF MEMS switches, which provide beneficial isolation and insertion loss properties that are advantageous for implementing in TTD applications. These RF MEMS switches use an electrically actuated mechanical movement to achieve an open circuit or a closed circuit in a RF transmission line. When the RF MEMS device is in an on position, the RF transmission line is “closed” and in the RF signal path. When the RF MEMS device is in an off position, the RF transmission line is “open,” and the RF transmission line is isolated from the RF signal path.
In TTD modules, RF MEMS switches may be actuated and de-actuated using an operational mode called hot switching. Hot switching occurs when an RF MEMS switch is actuated from the off position to the on position while a large voltage potential exists across the terminals of the RF MEMS switch or when an RF MEMS switch is de-actuated from the on position to the off position while a large current is flowing through the closed contacts of the RF MEMS switch. In either instance of hot switching, micro arcing occurs at the RF MEMS switch contacts, which exacerbates the degradation of the material and, therefore, the lifetime of the contacts. It is highly challenging and very costly to try to change the metallurgy or other structural features of an RF MEMS switch to improve hot switching performance. Thus, while hot switching allows a TTD module to remain “hot” and conduct an RF signal at all times, hot switching is still an undesirable mode of operation because of the increased risk of RF MEMS switch failure. This is particularly problematic for transmit arrays where the RF signals are often orders of magnitude higher in signal amplitude than in receive arrays.
RF MEMS switches can also be controlled according to another operational mode called cold switching. In cold switching, the RF input to the RF MEMS switches is shut off before the RF MEMS switches are actuated or de-actuated. Since the RF MEMS switches are operated when they are not conducting RF signals, operating the RF MEMS switches will not produce any micro arcing or significantly degrade the RF MEMS switches like operating the RF MEMS switches during hot switching. After the RF MEMS switches have been actuated or de-actuated, the RF signal is again supplied to the RF MEMS switches. Despite the added protection to RF MEMS switches provided by cold switching, cold switching is also undesirable because the RF input signal has to be entirely shut down for cold switching to be performed. Thus, the TTD module is not able to supply an output signal during the switching period. Moreover, this method would add substantial circuit complexity and cost as one has to selectively disable the RF path from each TTD module prior to changing the state of the TTD modules. A simpler approach would be to shut off the RF input to a large number of TTD modules and change the state of all the TTD modules at the same time. However, this approach would cause severe degradation of the antenna beam pattern.
Therefore, it would be desirable to actuate the switching elements of a TTD module using an operational mode that improves the reliability and lifetime of the switching elements without shutting down the input signal to the TTD module.